The present invention relates to measuring instruments of the type using a heated filament or filaments as sensing means, and more particularly, to an accurately regulated battery voltage supply and circuit condition indicator for use in such instruments.
Instruments of this type use an electrically-heated filament for sensing purposes and observe changes in electrical resistance of the filament to measure such quantities as concentration of combustible gases flowing over the filament. The filament is usually connected in a bridge circuit so that the change in resistance is detected as a voltage which can be amplified and applied to a suitable meter. The meter can be calibrated to read the desired quantity directly. To obtain the desired accuracy, the voltage applied to such filaments for heating them to operating temperature must be held at an accurately determined constant value, so that the measured bridge voltage reflects only changes in resistance of the filament and is not affected by random or uncontrolled changes or fluctuations in the applied voltage. When such a filament is initially energized, a certain time is required for the filament to become heated and for its temperature to stabilize at the normal operating level. Readings taken during this time may be erroneous or inconsistent and means for preventing use of the meter during such periods, or for indicating that the meter is not capable of use, are very desirable.